Uranium monosulfide-uranium monophosphide solid solutions



ug- 2, 1956 v vBAsKlN ETAL 3,264,223

as-a UP o A200 40o 600 600 /000 O Te mpefaufe) O INVENTORS yefzudaBsfzfz URANlUM MONOSULFDE-URANIUM MONO- PHOSPHDE SOLID SOLUTIONS YehudaBaskin, Chicago, and Peter D. Shalek, Champaign, lil., assignors to theUnited States of America as represented oy the United States AtomicEnergy Conlmission Filed .lune 2S, 1964, Ser. No. 378,490

4 Claims. (Cl. 252-3011) The invention described herein was made in thecourse of, or under, a contract with the United States Atomic EnergyCommission.

This invention relates to ceramic-type compositions of matter based onuranium compounds that are suitable as fuel for nuclear reactors.

Various high-melting uranium compounds have been investigated for use asnuclear reactor fuel. For instance, uranium monosuliide has beenconsidered for this purpose. The sulfide, however, has certaindrawbacks; it forms uranium oxysuliide, UOS, when heated in the presenceof air. Uranium oxysullide is rather volatile at elevated temperatures,which causes undesirably high losses; it also has a relatively lowmelting point which impairs its dimensional stability and has adisadvantageous effect on the sintering of the product. At l800 C., theusual sintering temperature, the oxysulfide is liquid.

Another compound considered for the above-described purpose is theuranium monophosphide. This compound oXi-dizes considerably more slowlythan the monosuliide, but it has some other drawbacks, namely it has atendency to crack on sintering. The slower oxidation of the uraniummonophosphide, it is believed, is due to the formation of a protectivefilm containing P205 and/ or uranium phosphate on the surface of themonophosphide.

It is an object of this invention to provide a ceramictype nuclearuranium fuel that does not crack when heated.

It is another object of this invention to provide a ceramic-type nuclearuranium fuel that has a relatively low vapor pressure at the elevatedtemperatures at which it is exposed in a nuclear reactor.

It is finally also an object of this invention to provide a ceramic-typenuclear uranium fuel that has a small grain size and ydoes not showgrain growth during use in the reactor.

It was found that a mixture of uranium monosulde and uraniummonophosphide is superior in many respects to either the monosulde orthe monophosphide alone. Uranium monosulde and uranium monophosphideform solid solutions with each other over the entire range ofcomposition in the form of a crystalline mass pertaining to theface-centered cubic system. It was found that the uranium sulfidecontent has an effect on the melting point of the composition and thatthe highest melting point, 2600 C., is obtained with a compositioncontaining about 30 w/o of uranium monosuliide. This melting point ishigher than either that of uranium monosulde (2450 C.) or of uraniummonophosphide (2540 C.). (All these melting points were determined in anatmosphere of hydrogen to avoid any contamination by anoxygen-containing product.) It was furthermore found that the uraniumsulfide content reduces the cracking tendency to a `minimum up to apercentage of 80% by weight. Uranium oxysulde is not formed incompositions that contain less than 80% of uranium monosultide; theycontain uranium dioxide as a second phase. Compositions having highercontents, however, do show uranium oxysulfide as a second phase. Uraniumdioxide is preferable as a second phase to uranium oxysuliide, becausethere it ties up a lesser weight of uranium than in the oxysulde;because it has a higher melting 3,264,223- Patented August 2, 1966 icepoint (2800 C.) than `the oxysulide (1900 C.); and because it does notlower the melting point of uranium monosullide as drastically as doesthe oxysulfide.

The product of this invention thus consists of a composition of matterformed of from 10 to 80% by weight of uranium monosulfde and from 90 to20% by weight of uranium monophosphide. The preferred composition, onaccount of a high melting point, is that containing from 20 to 40 w/o ofmonosulide, while the composition containing about 30% is the very best,as has been mentioned before.

Uranium monophosphide and uranium monosulfide can be made by any methodknown to those skilled in the ant; their preparation is not part of thisinvention. For instance, the phosphide can be made by the reaction ofphosphine, PH3, with uranium powder, the latter obtaiied by hydridingand dehydriding uranium metal; a temperature of approximately 385 C. issuitable for this reaction. A homogenization treatment is then appliedto the product consisting of heating at about 1400o C. in vacuum forabout two hours. The uranium monosulde can be prepared by the reactionof uranium powder and hydrogen sulfide at a temperature between 400 and550 C.; here, too, the product is advantageously homogenized by heatingfor about two hours at 1900 C. in vacuum or in an inert atmosphere, forinstance of argon gas.

The composition of matter of this invention is prepared by thoroughlymixing uranium monophosphide and uranium monosultide powders in theratio desired. While the particle size of the powders may vary widely, agrain size of about seven microns and about 21A. microns for the uraniumsulde and the uranium phosphide, respectively, have been used for thestudies leading to this invention.

If a fuel element of predetermined shape is to be formed from thecomposition of matter of this invention, a binder is preferably added tothe mixture. While any known binder that Idecomposes and/or volatilizeswhen heated can be used, the applicants found stearic acid particularlywell suitable; a quantity of about 1% by weight of the sulfide-phosphidemixture was satisfactory. The mass is then shaped into bodies of desiredconfiguration, and the bodies are heated at about 1800 C. in vacuum foraproximately two hours. At this temperature Ithe binder is completelydecomposed and/or volatilized, and the product consists `of a solidsolution of uranium monosulfide in uranium monophosphide or vice versa.

The specimens were examined metallographically, and the grain sizes,after two hours at 1800 C., were determined. Uranium monophosphide isfine-grained, uranium monosulide coarse-grained. The new compositionscontaining less than by weight of uranium monosuliide had anintermediate grain size, but much closer to that of uraniummonophosphide than lto that of uranium monosulde.

The volatility and hardness of the binary compositions also show valuesintermediate between those of the pure components.

The oxidation behavior of the new composition of matter was comparedwith that of the uranium monophosphide and that of the uraniummonosulfide by differential thermal analysis (DTA).

Curves were obtained automatically showing on the ordinate thetemperature difference between the specimen and a control sample and onthe abscissa the furnace temperature. These curves are shown in theaccompanying drawing. It will be obvious that uranium monosulfideexhibited a sharp exothermic peak at 380 C., while uranium monophosphideexhibited several broad peaks, the one at 560 C. being the mostpronounced. Addition of 20% of uranium monophosphi'de to uraniummonosulde shifted the exothermic peak of uranium monosulde'to 500 C.andbroladened -it considerably. A sample containing 40% of uraniummonophosphide showed the .major exotherrnic peak to be at 560 C., thesame temperature as for pure uranium fmonophosphide. A compositioncontaining 30% uranium monophosphide would be expected to have a peaksomewhere between 500 and 560 C. The higher the peak temperatures andthe less sharp the peaks are, the higher a resistance to oxidation isindicated.

It will be understood that the invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

What is claimed is:

1. As a new composition of matter, a sintered product formed of a solidsolution of uranium monosulde and uranium monophosphi-de.

2. The composition Aof matter of claim 1 wherein the solution consistsof from l0 to 80% by weight of ura- References Cited by the ExaminerUNITED STATES PATENTS 3,122,509 2/1964 Handwerk et al. 252-301.l X3,180,704 4/1965 Baskin 176-89 X 3,194,745 7/1965 Handwerk et al.252-30l.1 X

OTHER REFERENCES AEC Documents: ANL-6784, June 1964, page 40; ANL-6868,September 1964, page 156.

L. DEWAYNE RUTLEDGE, Primary Examiner.

1. AS A NEW COMPOSITION OF MATTER, A SINTERED PRODUCT FORMED OF A SOLIDSOLUTION OF URANIUM MONOSULFIDE AND URANIUM MONOPHOSPHIDE.